Support enabling/disabling a provider on a running trace

[daladim]

In ferrisetw 0.1, UserTrace::enable should be used to add a Provider to a trace. But, IIRC, this has no effect on a trace that has started already (the provider is pushed to a list, but EnableTraceEx2 is not called on this new trace).

To fix this problem, the next_major_version branch reworks this enable function and forces to define all Providers before the trace is started.

But the Windows API allows enabling/disabling providers when the trace is running (using EnableTraceEx2, so this could be worth supporting this in ferrisetw

[daladim]

Note: trace_callback_thunk is currently safe (memory-safety-wise) because CallbackData is not mutated when a trace is running.
When resolving this issue, we will mutate the list of providers (which is a member of CallbackData) in one thread while another thread may run a callback (and read this list of Providers). We must synchrionize it in some way and make sure the Provider cannot be removed during a callback (wrapping the Vec<Provider> in a Mutex may or may not be sufficient, this must be carefully checked)